Field of the Invention
At the present time, diverse applications are being used in which values, represented by a counter reading, for example, are recorded in a nonvolatile form. The values are preferably stored in read-only memories, such as EPROMs, EEPROMs and flash EPROMs, i.e. in memories which can be erased and can have information written to them again. The memories and the circuits determining the value, and also circuits that control the value determination and storage procedure, are usually produced in integrated circuits. The integrated circuits or the semiconductor chip that contains them are currently usually embedded in card-like data media for the purpose of better handling. In most cases, the value represents a monetary credit value that can only be reduced for example, in the case of telephone cards, but may also be increased again in the case of cash cards. Alternatively, the value may be the reading on an odometer in a vehicle, for example.
Published, Non-Prosecuted German Patent Application DE 36 38 505 A1 describes a value counter which is in the form of a volatile binary counter whose counter reading can be stored in nonvolatile form in an EEPROM after the data medium has been used. At the start of the data medium being used, the content of the EEPROM is transferred to the binary counter again as an initial counter reading. The counter can be operated only in the direction in which the value represented by the counter reading is reduced. Since the value of the data medium can thus not be increased, there is only a small incentive for manipulation by fraudulent parties.
A data medium described in International Patent Disclosure WO 96/10810 is intended to be rechargeable, and it should thus also be possible to increase its value. In this instance, the counter is formed using EEPROM memory cells, and is organized such that it operates as a multistage counter using the abacus principle. In order to rule out manipulation by changing the counter reading, two counters of identical construction are provided which are used alternately whenever the counter reading is increased. Although the counter configuration is very reliable, it requires a relatively large number of memory cells and thus has a high area requirement.
Published, European Patent Disclosure EP 0 398 545 A1 discloses an odometer which is likewise formed using a binary counter whose counter reading is stored in nonvolatile form in memory cells at regular intervals. Since it is not possible to erase and rewrite information to nonvolatile memory cells, particularly EEPROM cells, as often as desired, a number of identical memory areas are provided which are used successively, whenever the "life" of the previous memory area has come to an end. To distinguish the memory area that is current in each case, control memory cells are provided whose state is evaluated.
In the known counter, when there is a change of memory area, the new memory area is first erased. The current counter reading is then entered in the memory area. The associated control memory cell is subsequently programmed. Finally, the control memory cell for the old memory area is erased. A change of memory area requires a total of four erasing and programming operations to be carried out, which takes up a considerable amount of time. This presents a problem in an odometer in which a change of memory is necessary only after 20,000 km, for example.
This is not acceptable for telephone cards, however, in which each unit consumed necessitates a change of counter reading, and hence a change of memory area.